Calibration method for cooperative transmission of cell-free wireless network, and apparatus therefor

ABSTRACT

A method for calibration, in a serial fronthaul in which a calibration device and L (L is a natural number equal to or greater than 1) access node(s) (AN(s)) are serially connected, may include: transmitting, by the calibration device, to at least part of the L AN(s), a calibration command message indicating calibration of transmission paths; determining, by the calibration device and the at least part of the L AN(s), time delay values and phase characteristic values of the transmission paths of the at least part of the L AN(s); and transmitting, by the calibration device, to the at least part of the L AN(s), a calibration adjustment message indicating calibration of the transmission paths based on the time delay values and the phase characteristic values of the transmission paths of the at least part of the L AN(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Applications No.10-2020-0147179 filed on Nov. 5, 2020, and No. 10-2021-0140930 filed onOct. 21, 2021 with the Korean Intellectual Property Office (KIPO), theentire contents of which are hereby incorporated by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to a cooperative communication method ofdistributed base station apparatuses in a radio access network (RAN),and more particularly, to a method and an apparatus for radio frequency(RF) calibration of cooperative base station apparatuses, which arerequired to calculate accurate precoding (beamforming) matrixes fordownlink cooperative transmission based on uplink channel estimation ina cell-free massive multiple-input multiple-output (CFmMIMO) wirelesscommunication system.

2. Description of Related Art

In a cellular wireless communication system, each user terminal isgenerally provided with a service by one serving base station. However,in order to provide a consistent communication quality to all userterminals while accommodating the explosively increasing mobile traffic,all base stations around a user terminal need to provide services to theuser terminal without cell boundaries. Such the scheme may be defined as‘cell-free wireless communication’.

In such the cell-free wireless communication, user terminals maysynchronously and simultaneously transmit their own pilot signals to allbase stations according to a time division duplex (TDD) protocol. Eachbase station may estimate uplink channel coefficients between itself andeach user terminal through the pilot signal, and may utilize TDD channelreciprocity to estimate downlink channel coefficients between itself andeach user terminal. Each base station may estimate downlink channelinformation based on uplink channel information without feedback ofchannel state information from the terminal through the above-describedTDD channel reciprocity. All base stations around the terminal mayacquire downlink channel information with the terminal in theabove-described manner, and based on the downlink channel information,precode and transmit a signal for the terminal. Accordingly, since theterminal can simultaneously receive the precoded signal based ondownlink channel information from all nearby base stations no matterwhere the terminal is located, the terminal can receive a high-qualitysignal without interference with other terminals. Such the scheme inwhich a plurality of base stations cooperate to perform transmission toa specific terminal may be referred to as ‘coherent joint transmission(hereinafter, ‘C-JT’). In the cell-free wireless communication, the C-JTcooperative transmission scheme may be basically used.

As described above, in order to efficiently perform the C-JT cooperativetransmission scheme in the cell-free wireless communication,bidirectional (i.e., uplink and downlink) channel information should beacquired from unidirectional (especially, uplink) channel information byutilizing the TDD channel reciprocity. In order to guarantee the TDDchannel reciprocity, a characteristic difference (mismatch) of RFtransmission/reception paths should be calibrated.

SUMMARY

Accordingly, exemplary embodiments of the present disclosure aredirected to providing a method of performing calibration on a mismatchbetween transmission paths in a serial fronthaul where a calibrationdevice and L access nodes (ANs) are serially connected.

Accordingly, exemplary embodiments of the present disclosure aredirected to providing a method of performing calibration on a mismatchbetween reception paths in a serial fronthaul where a calibration deviceand L ANs are serially connected.

Accordingly, exemplary embodiments of the present disclosure aredirected to providing a method of performing calibration on a timedivision duplex (TDD) channel reciprocity of each AN in a serialfronthaul where a calibration device and L ANs are serially connected.

According to a first exemplary embodiment of the present disclosure, amethod for calibration in a serial fronthaul in which a calibrationdevice and L (L is a natural number equal to or greater than 1) accessnode(s) (AN(s)) are serially connected may comprise: a step (a) oftransmitting, by the calibration device, to at least part of the LAN(s), a calibration command message indicating calibration oftransmission paths; a step (b) of determining, by the calibration deviceand the at least part of the L AN(s), time delay values and phasecharacteristic values of the transmission paths of the at least part ofthe L AN(s); and a step (c) of transmitting, by the calibration device,to the at least part of the L AN(s), a calibration adjustment messageindicating calibration of the transmission paths based on the time delayvalues and the phase characteristic values of the transmission paths ofthe at least part of the L AN(s).

The step (b) may further comprise: a step (b-1) of transmitting, by anl-th AN of the at least part of the L AN(s), a transmission pathcalibration signal to the calibration device through each of N_(l)transmission paths included in the l-th AN; a step (b-2) of determining,by the calibration device, a time delay value of an i-th transmissionpath among the N_(l) transmission paths based on a difference between atransmission timing of the transmission path calibration signal at thel-th AN and a reception timing of the transmission path calibrationsignal at the calibration device; and a step (b-3) of determining, bythe calibration device, a phase characteristic value of the i-thtransmission path among the N_(l) transmission paths based on adifference between a phase of the transmission path calibration signalat the transmission timing and a phase of the transmission pathcalibration signal at the reception timing.

The step (b) may further comprise: a step (b-4) of reporting, by thel-th AN, time delay values and phase characteristic values of the N_(l)transmission paths to the calibration device; and a step (b-5) ofdetermining, by the calibration device, the time delay values and thephase characteristic values of the transmission paths of the at leastpart of the L AN(s) through the steps (b-1) to (b-4).

The transmission path calibration signals may be sequentiallytransmitted at a time interval Δ_(tx) after a predetermined timeΔ_(start) elapses from a transmission timing t₀ of the calibrationcommand message.

The step (c) may further comprise: a step (c-1) of determining, by thecalibration device, a difference between a maximum time delay valueamong the time delay values of the transmission paths of the at leastpart of the L AN(s) and a time delay value of an i-th transmission pathof an l-th AN of the at least part of the L AN(s) as a time delaycalibration value for the i-th transmission path; a step (c-2) ofdetermining, by the calibration device, a difference between a maximumphase characteristic value among the phase characteristic values of thetransmission paths of the at least part of the L AN(s) and a phasecharacteristic value of the i-th transmission path of the l-th AN of theat least part of the L AN(s) as a phase characteristic calibration valuefor the i-th transmission path; and a step (c-3) of transmitting, by thecalibration device, the time delay calibration value and the phasecharacteristic calibration value of the i-th transmission path to thel-th AN through a calibration adjustment message.

The serial fronthaul may comprise: a timing clock signal line for clocksynchronization between the calibration device and the L AN(s); acalibration signal line for transferring a calibration signal betweenthe calibration device and the L AN(s); and a data and control signalline for transferring data and control signals for calibration betweenthe calibration device and the L AN(s).

The calibration device may exist as a separate hardware device, one ofthe L AN(s) may serve as the calibration device, or a central processor(CP) connected to the L AN(s) through the serial fronthaul may perform arole of the calibration device.

According to a second exemplary embodiment of the present disclosure, amethod for calibration in a serial fronthaul in which a calibrationdevice and L (L is a natural number equal to or greater than 1) AN(s)are serially connected may comprise: a step (a) of transmitting, by thecalibration device, to at least part of the L AN(s), a calibrationcommand message indicating calibration of reception paths; a step (b) ofdetermining, by the calibration device and the at least part of the LAN(s), time delay values and phase characteristic values of thereception paths of the at least part of the L AN(s); and a step (c) oftransmitting, by the calibration device, to the at least part of the LAN(s), a calibration adjustment message indicating calibration of thereception paths based on the time delay values and the phasecharacteristic values of the reception paths of the at least part of theL AN(s).

The step (b) may further comprise: a step (b-1) of transmitting, by thecalibration device, a reception path calibration signal to an l-th AN ofthe at least part of the L AN(s) through each of N_(l) reception pathsincluded in the l-th AN; a step (b-2) of determining, by the l-th AN, atime delay value of an i-th reception path among the N_(l) receptionpaths based on a difference between a transmission timing of thereception path calibration signal at the calibration device, which isreceived through the i-th reception path, and a reception timing of thereception path calibration signal at the calibration device; a step(b-3) of determining, by the l-th AN, a phase characteristic value ofthe i-th reception path among the N_(l) reception paths based on adifference between a phase of the reception path calibration signal atthe transmission timing and a phase of the reception path calibrationsignal at the reception timing; and a step (b-4) of determining, by thel-th AN, the time delay values and the phase characteristic values ofthe N_(l) reception paths through the steps (b-1) to (b-3), andreporting the time delay values and the phase characteristic values ofthe N_(l) reception paths to the calibration device.

When the calibration command message indicates a number R of repeatedtransmissions of the reception path calibration signal, the receptionpath calibration signal may be repeatedly transmitted R times throughthe i-th reception path, and the time delay value and the phasecharacteristic value for the i-th reception path may be determined basedon R repeated transmissions of the reception path calibration signal.

The reception path calibration signals may be sequentially transmittedat a time interval Δ_(rx) after a predetermined time Δ_(start) elapsesfrom a transmission timing t₀ of the calibration command message.

The step (c) may further comprise: a step (c-1) of determining, by thecalibration device, a maximum time delay value among the time delayvalues of the reception paths of the at least part of the L AN(s) and amaximum phase characteristic value among the phase characteristic valuesof the reception paths of the at least part of the L AN(s); a step (c-2)of determining, by the calibration device, a difference between themaximum time delay value and a maximum time delay value of receptionpaths of an l-th AN of the at least part of the L AN(s) as a time delaycalibration value for the l-th AN; a step (c-3) of determining, by thecalibration device, a difference between the maximum phasecharacteristic value and a maximum phase characteristic value of thereception paths of the l-th AN of the at least part of the L AN(s) as aphase characteristic calibration value for the l-th AN; and a step (c-4)of transmitting, by the calibration device, the time delay calibrationvalue and the phase characteristic calibration value to the l-th ANthrough a calibration adjustment message.

The serial fronthaul may comprise: a timing clock signal line for clocksynchronization between the calibration device and the L AN(s); acalibration signal line for transferring a calibration signal betweenthe calibration device and the L AN(s); and a data and control signalline for transferring data and control signals for calibration betweenthe calibration device and the L AN(s).

The calibration device may exist as a separate hardware device, one ofthe L ANs may serve as the calibration device, or a central processor(CP) connected to the L AN(s) through the serial fronthaul may perform arole of the calibration device.

According to a third exemplary embodiment of the present disclosure, amethod for calibration in a serial fronthaul in which a calibrationdevice and L (L is a natural number equal to or greater than 1) AN(s)are serially connected may comprise: a step (a) of transmitting, by thecalibration device, to an l-th AN of the L AN(s), a calibration commandmessage indicating calibration of a TDD channel reciprocity; a step (b)of measuring, by the calibration device and the l-th AN, gaincharacteristic values of N_(l) reception paths and N_(l) transmissionpaths included in the l-th AN; and a step (c) of determining, by thecalibration device, a gain calibration value for each of the N_(l)reception paths based on the gain characteristic values of the N_(l)reception paths and the N_(l) transmission paths, and transmitting acalibration adjustment message including the gain calibration value tothe l-th AN.

The step (b) may comprise: transmitting, by the calibration device, tothe l-th AN a TDD channel reciprocity calibration signal through theN_(l) reception paths included in the l-th AN; reporting, by the l-thAN, a reception signal gain measurement value of the TDD channelreciprocity calibration signal for each of the N_(l) reception paths;transmitting, by the l-th AN, to the calibration device a TDD channelreciprocity calibration signal through the N_(l) transmission pathsincluded in the l-th AN; and measuring, by the calibration device, areception signal gain measurement value of the TDD channel reciprocitycalibration signal for each of the N_(l) transmission paths.

The TDD channel reciprocity calibration signals may be sequentiallytransmitted at a time interval Δ_(tx) after a predetermined timeΔ_(start) elapses from a transmission timing t₀ of the calibrationcommand message or a timing of reporting the reception signal gainmeasurement value.

The serial fronthaul may comprise: a timing clock signal line for clocksynchronization between the calibration device and the L AN(s); acalibration signal line for transferring a calibration signal betweenthe calibration device and the L AN(s); and a data and control signalline for transferring data and control signals for calibration betweenthe calibration device and the L AN(s).

The calibration device may exist as a separate hardware device, one ofthe L ANs may serve as the calibration device, or a central processor(CP) connected to the L AN(s) through the serial fronthaul may perform arole of the calibration device.

Downlink channel information for the L AN(s) may be estimated fromuplink channel information for the L AN(s) using the TDD channelreciprocity.

According to exemplary embodiments of the present disclosure, a mismatchbetween a plurality of RF transmission/reception paths can be corrected(calibrated) without wasting radio resources in a serial fronthaulenvironment where distributed ANs are connected through a single cable.In particular, a more accurate mismatch calibration can be performedwhile maintaining the simple cabling advantage of the serial fronthaul.Through this, bidirectional (uplink and downlink) channels can beestimated through unidirectional (especially, uplink) channelmeasurement, and services can be provided to terminals throughcooperative transmission between the ANs, resulting in higher qualitydata transmission.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating a configuration of acell-free massive MIMO (CFmMIMO) system to which exemplary embodimentsof the present disclosure are applied.

FIG. 2 is a conceptual diagram illustrating a configuration of acell-free massive MIMO system using a serial fronthaul to whichexemplary embodiments of the present disclosure are applied.

FIG. 3 is a conceptual diagram illustrating a configuration of a serialfronthaul system to which a serial fronthaul calibration methodaccording to an exemplary embodiment of the present disclosure isapplied.

FIG. 4 is a conceptual diagram illustrating a mismatch adjustment blockaccording to an exemplary embodiment of the present disclosure.

FIGS. 5A and 5B are sequence charts illustrating a method of calibratingmismatch of transmission paths and/or reception paths in a serialfronthaul according to an exemplary embodiment of the presentdisclosure.

FIG. 6A is a flowchart illustrating an operation of a calibrationcontroller in the method of calibrating a mismatch of transmission pathsaccording to FIG. 5A.

FIG. 6B is a flowchart illustrating an operation of a calibration blockin the method of calibrating a mismatch of transmission paths accordingto FIG. 5A.

FIG. 7A is a flowchart illustrating an operation of a calibrationcontroller in the method of calibrating a mismatch of reception pathsaccording to FIG. 5B.

FIG. 7B is a flowchart illustrating an operation of a calibration blockin the method of calibrating a mismatch of reception paths according toFIG. 5B.

FIG. 8A is a sequence chart illustrating a method of performingcalibration for TDD channel reciprocity in a serial fronthaul accordingto an exemplary embodiment of the present disclosure.

FIG. 8B is a flowchart illustrating an operation of a calibrationcontroller in a method of performing calibration for TDD channelreciprocity in a serial fronthaul according to an exemplary embodimentof the present disclosure.

FIG. 9 is a conceptual diagram illustrating a connection structure of aserial fronthaul according to an exemplary embodiment of the presentdisclosure.

FIG. 10 is a conceptual diagram illustrating a hierarchical calibrationmethod according to an exemplary embodiment of the present disclosure ina system including a plurality of serial fronthauls.

FIG. 11 is a conceptual diagram illustrating a hierarchical calibrationmethod according to another exemplary embodiment of the presentdisclosure in a system including a plurality of serial fronthauls.

FIG. 12 is a block diagram illustrating a configuration of acommunication node according to exemplary embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present disclosure are disclosed herein.However, specific structural and functional details disclosed herein aremerely representative for purposes of describing embodiments of thepresent disclosure. Thus, embodiments of the present disclosure may beembodied in many alternate forms and should not be construed as limitedto embodiments of the present disclosure set forth herein.

Accordingly, while the present disclosure is capable of variousmodifications and alternative forms, specific embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit the present disclosure to the particular forms disclosed, but onthe contrary, the present disclosure is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of thepresent disclosure. Like numbers refer to like elements throughout thedescription of the figures.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(i.e., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentdisclosure. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this present disclosure belongs.It will be further understood that terms, such as those defined incommonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein.

A communication system to which exemplary embodiments according to thepresent disclosure are applied will be described. The communicationsystem to which the exemplary embodiments according to the presentdisclosure are applied is not limited to the contents described below,and the exemplary embodiments according to the present disclosure may beapplied to various communication systems. Here, a communication systemmay be used in the same meaning as a communication network.

Hereinafter, preferred exemplary embodiments of the present disclosurewill be described in greater detail with reference to the accompanyingdrawings. In order to facilitate general understanding in describing thepresent disclosure, the same components in the drawings are denoted withthe same reference signs, and repeated description thereof will beomitted.

FIG. 1 is a conceptual diagram illustrating a configuration of acell-free massive MIMO (CFmMIMO) system to which exemplary embodimentsof the present disclosure are applied.

Referring to FIG. 1, a cell-free massive MIMO system 100 may include onecentral processor (CP) 110, a plurality of distributed access nodes(ANs) 120, and a plurality of distributed terminals (e.g., userequipments (UEs)) 130. The functions of such the devices may bedescribed as follows in terms of downlink cooperative transmission.

One CP 110 may be connected to the distributed ANs 120 through fronthaullinks, and may transmit data for all the terminals 130 to the ANs 120through the fronthaul links. In addition, the CP 110 may receive channelstatistics information of all the terminals 130 from the distributedANs, and may control a transmission power used by a specific AN toprovide services to a specific terminal. Each AN may be connected to theCP 110 by a fronthaul link, and each AN may include one or more antennasand RF devices, and may include a part or all of baseband functions.Each AN may cooperate with other ANs to provide services to terminals.The terminal may transmit a pilot signal allocated thereto to all ANsthrough uplink, so that the ANs can acquire downlink channel informationthrough channel reciprocity. In addition, the terminal may receive C-JTsignals including downlink data from all the ANs.

In order for the C-JT cooperative transmission used in the cell-freewireless communication to provide high-quality signals to a terminal,accurate synchronization between the distributed ANs should bemaintained, and an array antenna of a specific AN and a plurality oftransmission/reception RF paths connected thereto should have the sametime delay and phase shift values. In addition, for the C-JT cooperativetransmission, transmission and reception RF paths should have the sametime delay and phase shift values even between the ANs. Meanwhile, inthe C-JT cooperative transmission used in the cell-free wirelesscommunication, downlink channel information may be obtained from uplinkchannel information by utilizing the TDD channel reciprocity. The uplinkchannel information may include transmission (TX) RF path characteristicinformation of a terminal, uplink radio channel information, andreception (RX) RF path characteristic information of a base station.Downlink channel information applied to downlink data transmission mayinclude TX RF path characteristic information of the base station,downlink radio channel information, and RX RF path characteristicinformation of the terminal. Accordingly, in order to apply the TDDchannel reciprocity, a characteristic difference (mismatch) of RFtransmission/reception paths should be corrected (calibrated).

The function of correcting the characteristic difference of theabove-described RF transmission/reception paths may be referred to as‘calibration’. As the use of multiple antennas and MIMO transmissionschemes are standardized, calibration functions have been developedtogether. An RF transmission path may include a digital-to-analogconverter (DAC), up converter, power amplifier, and filter, and an RFreception path may include a duplexer (duplexer), low-noise amplifier(LNA), down converter, filter, and analog-to-digital converter (ADC).The RF transmission path and the RF reception path may be generallyconnected to one antenna through a switch, and a transmission functionand a reception function may be performed as being temporally separatedaccording to the TDD operation. Since individual paths include differentcomponents, each path may exhibit different delay characteristics, phasecharacteristics, and gain characteristics. The conventional calibrationtechniques are limited to calibration that adjusts mismatches of aplurality of transmission/reception paths due to the use of multipleantennas within one base station. Accordingly, there is a need for acalibration technique for mismatches of transmission/reception pathsbetween a plurality of ANs to cost-effectively support the C-JTcooperative transmission in the cell-free wireless communication.

The conventional cell-free MIMO system requires a dedicated fronthaullink for each of the ANs as shown in FIG. 1. Each AN may pre-process asignal (e.g., pilot signal) received on the uplink, calculate a channelestimate, and deliver the calculated channel estimate to the CP over itsdedicated fronthaul link. Also, the CP may generate a precoded signalbased on the received channel estimates, and deliver the precoded signalto each AN. The above-described architecture may be preferable from theviewpoint of communication performance, but has a problem from theviewpoint of costs because a large number of long cables are required.

FIG. 2 is a conceptual diagram illustrating a configuration of acell-free massive MIMO system using a serial fronthaul to whichexemplary embodiments of the present disclosure are applied.

Referring to FIG. 2, in order to enable a more practical architectureand distributed processing, a CFmMIMO system 200, where one fronthaulcable connects multiple ANs in series in a daisy-chain manner to reducecabling, is shown.

However, calibration techniques for resolving a problem of mismatches oftransmission/reception paths within the same AN andtransmission/reception paths between distributed ANs and a problem ofTDD channel non-reciprocity due to a mismatch betweentransmission/reception paths with the same transceiver have not beenconsidered.

Configuration of a Serial Fronthaul Calibration System

FIG. 3 is a conceptual diagram illustrating a configuration of a serialfronthaul system to which a serial fronthaul calibration methodaccording to an exemplary embodiment of the present disclosure isapplied.

Referring to FIG. 3, a serial fronthaul 310 may connect L (L is anatural number greater than or equal to 1) AN(s) (e.g., 320 and 330) inseries. Each AN #1 connected to the serial fronthaul 310 (hereinafter,the AN #l is exemplified with the AN #1) may include a plurality ofantennas (e.g., N_(l) antennas), a switch 321 (hereinafter, ‘TDD switch(S_(TDD))’) for switching the antennas between a transmission path and areception path according to a TDD protocol, transceivers 322 (TX i andRX i, i=1, . . . , N_(l)) connected to corresponding antennas throughthe switch 321, mismatch adjustment blocks 323 adjusting the mismatchesbetween the transceivers, and a signal processing module 324 performinginterface and signal processing functions with a CP 340.

The signal processing module 324 may include a local clock 324-1 thatprovides timing for internal operations of the AN #l, a calibrationblock 324-2 to be described later, and a baseband function block 324-3functionally split from the CP 340. In addition, the AL #l may furtherinclude a switch 325 (hereinafter, referred to as ‘CAL switch(S_(CAL))’) for connecting a calibration signal line 311 of thefronthaul 310 to the transmission path and reception path of thetransceiver 322 in conjunction with the TDD switch 321. Furtherdescribing the interworking of the TDD switch 321 and the CAL switch325, when the antennas are used for transmission according to the TDDprotocol, the TDD switch 321 may connect the antennas with TX parts ofthe transceivers 322, and the CAL switch 325 may connect the calibrationsignal line 311 to RX parts of the corresponding transceivers 322. Onthe other hand, when the TDD switch 321 connects the antennas to the RXparts of the transceivers 322, the CAL switch 325 may connect thecalibration signal line 311 to the TX parts of the transceivers 322.

As shown in FIG. 3, the plurality of ANs each including theabove-described components may be connected with each other through adata and control signal line 312, a timing clock signal line 313, andthe calibration signal line 311 included in the serial fronthaul 310.Further, the plurality of ANs may be also connected to the CP 340through the serial fronthaul 310. In the following exemplary embodimentsrelating to calibration in the serial fronthaul, the data and controlsignal line 312 may be used for exchanging information for thecalibration among the CP 340, a calibration controller 355 in acalibration device 350, and the calibration blocks 324-2 in theindividual AN. The timing clock signal line 313 may connect a masterclock 341 of the CP 340 that provides a reference clock and the internallocal clocks 324-1 of the distributed ANs to support synchronizationtherebetween. For the calibration signal line 311, a circuit length froma CAL switch 351 of the calibration device 350 to a point J₀, a circuitlength from the CAL switch 325 of each antenna of the AN #1 320 to apoint J₁, and a circuit length from the CAL switch 325 of each antennaof the AN #l (l=2, . . . , L) to a point J_(l) may all be designed tohave exact fixed lengths. In the following exemplary embodiments, it isassumed that the circuit lengths are all the same. Although notseparately mentioned below, an additional calibration circuit may beused to assume the same circuit length.

As described above, the serial fronthaul according to an exemplaryembodiment of the present disclosure may include the calibration device350 for adjusting RF mismatches of a series of ANs connected by theserial fronthaul. Although the exemplary embodiment in which thecalibration device 350 exists as an independent device is shown in FIG.3, exemplary embodiments in which a specific AN performs the role of thecalibration device or the CP 340 performs the role of the calibrationdevice are also possible. In the following description, an exemplaryembodiment in which one independent calibration device 350 is includedin the serial fronthaul is considered.

The calibration device 350 may include the CAL switch 351 for connectinga calibration transceiver 352 to the calibration signal line 311 of theserial fronthaul 310 according to the TDD protocol, a transceiver 352comprising a calibration TX part (CAL TX part) for transmitting acalibration signal (e.g., calibration waveform signal) and a calibrationRX part (CAL RX part) for receiving a calibration signal, a mismatchadjustment block 353 for adjusting a mismatch between the TX/RX parts ofthe calibration transceiver, a local clock 354 that provides timing t₀the components of the calibration device, and a calibration controller355 to be described later.

FIG. 4 is a conceptual diagram illustrating a mismatch adjustment blockaccording to an exemplary embodiment of the present disclosure.

Referring to FIG. 4, a configuration of a mismatch adjustment block 323for adjusting a mismatch of the i-th (i=1, . . . , N_(l)) transceiver inthe AN #l (l=1, . . . , L) having N_(l) antennas is shown. That is, theN_(l) transceivers and the mismatch adjustment blocks may have aone-to-one correspondence.

Each mismatch adjustment block may include a transmission delay adjusterτ_(tx) ^(i,l) and a transmission phase adjuster ϕ_(tx) ^(i,l) forcalibrating a time delay and a phase of a transmission path of acorresponding transceiver, a reception delay adjuster τ_(rx) ^(i,l) anda reception phase adjuster ϕ_(rx) ^(i,l) for calibrating a time delayand a phase of a reception path of the transceiver, and a gain adjusterC_(Rec) ^(i,l) for adjusting a channel non-reciprocity between thetransmission path and the reception path of the transceiver.

Calibration values for the mismatch adjustment blocks of the transceivermay be provided from the calibration block 324-2 of the corresponding ANthrough cooperation with the calibration device 350. In an exemplaryembodiment, the mismatch adjustment block 323 may be implemented as anadditional HW block. However, exemplary embodiments in which themismatch adjustment block 323 is implemented in software within thebaseband function block 324-3 are also possible.

Hereinafter, with reference to FIGS. 3 and 4, methods for calibratingthe time delay and phase mismatch of transmission/reception paths of aplurality of transceivers within an AN, the time delay and phasemismatch of transmission/reception paths of a plurality of transceiversof ANs, and the TDD channel non-reciprocity mismatch of a transceiverwill be described. In the following description, it is assumed that themaster clock 341 of the CP 340 is accurately synchronized with the localclocks of the calibration device 350 and all the ANs 320 and 330. Also,message delivery over the data and control signal line 312 may beassumed to have no error. The calibration controller 355 of thecalibration device 350 may instruct the calibration to be performed bydesignating a specific transceiver of a specific AN connected to theserial fronthaul through the serial fronthaul data and control signalline 312 at an arbitrary time point, or instruct the calibration to beperformed for all transceivers of all ANs.

Transmission/Reception Path Calibration

FIGS. 5A and 5B are sequence charts illustrating a method of calibratingmismatch of transmission paths and/or reception paths in a serialfronthaul according to an exemplary embodiment of the presentdisclosure.

In FIG. 5A, a procedure of exchanging messages and calibration signalsfor calibrating a mismatch of transmission paths between the calibrationcontroller 355 of the calibration device 350 and the calibration block324-2 of the AN #l is shown. In addition, in FIG. 5B, a procedure forexchanging messages and calibration signals for calibrating a mismatchof reception paths between the calibration controller 355 of thecalibration device 350 and the calibration block 324-2 of the AN #l isshown.

Referring to FIG. 5A, in order to initiate measurement of time delay andphase characteristics of N_(l) transmission paths of the AN #l, thecalibration controller 355 may transmit a calibration command message(AID=l, DIR=Tx) to the calibration block 324-2 of the AN #l through thedata and control signal line 312 of the serial fronthaul 310 (S510).Here, the AID may indicate an identifier (i.e., AN ID) of the AN, andthe DIR may indicate target paths (i.e., transmission paths or receptionpaths) for measuring a mismatch. In the exemplary embodiment of FIG. 5A,the DIR may be set to ‘Tx’, which means that the targets of mismatchcalibration are the transmission paths. The calibration block 324-2 ofthe AN #l may transmit a transmission path calibration signal (i.e., TXCAL waveform) after Δ_(start) elapses from a timing t₀ at which thecalibration command message is transmitted (S521). Since the clocks ofthe calibration block 324-2, which is an entity transmitting thetransmission path calibration signal, and the calibration controller355, which is an entity receiving the transmission path calibrationsignal, are in a synchronized state, the calibration controller 355 maytransmit the timing t₀ in form of a time stamp by including it in thecalibration command message, and the calibration block 324-2 maytransmit the transmission path calibration signal on a transmission path1 (i.e., TX 1) at the timing t₀+Δ_(start) (S521). The calibration block324-2 may sequentially transmit a total of N_(l) transmission pathcalibration signals at an interval of Δ_(tx) on the paths TX 1, TX 2, .. . , and TX N_(l) (S521 to S524). After transmitting the lasttransmission path calibration signal (S524), the calibration block 324-2may notify that transmission of the transmission path calibrationsignals has been completed by transmitting a calibration completionreport (e.g. CAL report (complete)) message to the calibrationcontroller 355 through the data and control signal line 312 (S530). Onthe other hand, the calibration controller 355 should make a timeinterval from the timing t₀+Δ_(start) to a timingt₀+Δ_(start)+(N_(l)−1)Δ_(tx) become a radio reception operation periodof the AN #l according to the TDD protocol. A time delay value τ_(i,l)^(TX) of the transmission path calibration signal transmitted throughthe path TX i (1≤i≤N_(l)) of the AN #l may be defined as a differencebetween a transmission timing t₀+Δ_(start)+(i−1)Δ_(tx) of acorresponding transmission path calibration signal at the calibrationblock 324-2 and a reception timing of the transmission path calibrationsignal at the calibration controller 355, and may be expressed asEquation 1 below according to the transmission path of the transmissionpath calibration signal.τ_(i,l) ^(TX)=τ_(TX i,l)+τ_(J) _(l) ^(TX)+τ_(J) _(l) _(-J) ₀ ^(TX)+τ_(J)₀ ^(TX)+τ_(CAL RX)  [Equation 1]

In Equation 1, τ_(TX i,l) may be the time delay value of the path TX iof the AN #l that is the calibration target, τ_(J) _(l) ^(TX) may be atime delay value from a point of S_(CAL) of the AN #l to a point J_(l),τ_(J) _(l) _(-J) ₀ ^(TX) may be a time delay value from the point J_(l)to a point J₀, τ_(J) ₀ ^(TX) may be a time delay value from a point ofS_(CAL) of the calibration device 350 to the point J₀, and τ_(CAL RX)may be a time delay value of the RX path of the calibration device 350.The above-described τ_(J) _(l) ^(TX), τ_(J) ₀ ^(TX), and τ_(J) _(l)_(-J) ₀ ^(TX) may be well-known fixed values, and T_(CAL RX) may be acomponent value of a common path for measurement of τ_(i,l) ^(TX), ∀i,∀l. Accordingly, the value of τ_(TX i,l) may be derived from themeasured value of τ_(i,l) ^(TX).

That is, the phase characteristic value of the path TX i (1≤i≤N_(l)) ofthe AN #l may be determined based on a difference between a phase at thetransmission timing and a phase at the reception timing of thetransmission path calibration signal transmitted through the path TX i.In addition, the phase measurement value ϕ_(i,l) ^(TX) of thetransmission path calibration signal transmitted through the path TX i(1≤i≤N_(l)) of the AN #l may be expressed as Equation 2 below accordingto the transmission path of the transmission path calibration signal.ϕ_(i,l) ^(TX)=ϕ_(TX i,l)+ϕ_(J) _(l) ^(TX)+ϕ_(J) _(l) _(-J) ₀ ^(TX)+ϕ_(J)₀ ^(TX)+ϕ_(CAL RX)  [Equation 2]

In Equation 2, ϕ_(TX i,l) may be the phase characteristic value of thepath TX i of the AN #l that is the calibration target, ϕ_(J) _(l) ^(TX)may be a phase characteristic value from the point of S_(CAL) of the AN#l to the point J_(l), ϕ_(J) _(l) _(-J) ₀ ^(TX) may be a phasecharacteristic value from the point J_(l) to the point J₀, ϕ_(J) ₀ ^(TX)may be a phase characteristic value from the point of S_(CAL) of thecalibration device 350 to the point J₀, and ϕ_(CAL RX) may be a phasecharacteristic value of the RX path of the calibration device 350. Theabove-described ϕ_(J) _(l) ^(TX), ϕ_(J) ₀ ^(TX), and ϕ_(J) _(l) _(-J) ₀^(TX) may be well-known fixed values, and ah ϕ_(CAL RX) may be acomponent value of a common path for measurement of ϕ_(i,l) ^(TX), ∀i,∀l. Accordingly, the value of ϕ_(TX i,l) may be derived from themeasured value of ϕ_(i,l) ^(TX). The calibration controller 355 maycomplete the measurement of the time delay values ϕ_(TX i,l) (1≤i≤N_(l))and the phase characteristic values ϕ_(TX i,l) (1≤i≤N_(l)) of the N_(l)transmission paths of the AN #l by receiving the calibration completionreport (i.e., report (complete)) message from the calibration block324-2 of the AN #l.

The calibration controller 355 of the calibration device 350 may performthe above-described procedure with each of the other ANs (S540). Aftercompleting mismatch measurement for a total of Σ_(l=1) ^(L) N_(l)transmission paths for a total of the L ANs included in the serialfronthaul, the maximum time delay value max_(i,m)τ_(TX i,m) and themaximum phase characteristic value max_(i,m)ϕ_(TX i,m) may be derived.The calibration controller 355 may transmit, to the calibration block324-2 of the AN #l, a calibration adjustment message including timedelay and phase characteristic calibration values τ_(tx) ^(i,l)ϕ_(tx)^(i,l) i=1, . . . , N_(l) of the N_(l) transmission paths of the AN #l(S550). The time delay and phase characteristic calibration valuesτ_(tx) ^(i,l), τ_(tx) ^(i,l) for the i-th transmission path of the AN #lmay be expressed as Equation 3 below.

$\begin{matrix}{{\tau_{tx}^{i,l} = {{\max\limits_{i,m}\tau_{{TXi},m}} - \tau_{{TXi},l}}}{\phi_{tx}^{i,,l} = {{\max\limits_{i,m}\phi_{{TXi},m}} - \phi_{{TXi},l}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Referring to FIG. 5B, in order to initiate measurement of time delay andphase characteristics of N_(l) reception paths of the AN #l, thecalibration controller 355 may transmit a calibration command message(AID=l, DIR=Rx, REP=n) to the calibration block 324-2 of the AN #lthrough the data and control signal line 312 of the serial fronthaul 310(S550). Here, the AID may indicate an identifier (i.e., AN ID) of theAN, and the DIR may indicate target paths (i.e., transmission paths orreception paths) for measuring a mismatch. In the exemplary embodimentof FIG. 5B, the DIR may be set to ‘Rx’, which means that the targets ofmismatch calibration are the reception paths. In addition, the REP maymean the number of times of transmission of a reception path calibrationsignal (i.e., RX CAL waveform) for mismatch measurement. The calibrationcontroller 355 may transmit the reception path calibration signal (i.e.,RX CAL waveform) after Δ_(start) elapses from a timing t₀ at which thecalibration command message is transmitted (S561). Since the clocks ofthe calibration block 324-2, which is an entity receiving the receptionpath calibration signal, and the calibration controller 355, which is anentity transmitting the reception path calibration signal, are in asynchronized state, the calibration controller 355 may transmit thetiming t₀ in form of a time stamp by including it in the calibrationcommand message, and may transmit the reception path calibration signalon the calibration TX path at the timing t₀+Δ_(start). The calibrationcontroller 355 may sequentially transmit a total of (REP=n) receptionpath calibration signals at an interval of Δ_(rx) on the samecalibration TX path (S561 to S562). On the other hand, the calibrationcontroller 355 should allow the repeatedly-transmitted reception pathcalibration signals to be received on the reception path of the AN #l ina radio transmission operation period of the AN #l according to the TDDprotocol. A time delay value τ_(i,l) ^(RX) of the n-th (1≤n≤Rep)reception path calibration signal received through the path RXi(1≤i≤N_(l)) of the AN #l may be defined as a difference between atransmission timing t₀+Δ_(start)+(n−1)Δ_(rx) of a correspondingreception path calibration signal at the calibration controller 355 anda reception timing of the reception path calibration signa at thecalibration block 325-4, and may be expressed as Equation 4 belowaccording to the reception path of the reception path calibrationsignal.τ_(i,l) ^(RX)=τ_(RX i,l)+τ_(J) _(l) ^(RX)+τ_(J) _(l) _(-J) ₀ ^(RX)+τ_(J)₀ ^(RX)+τ_(CAL TX)  [Equation 4]

In Equation 4, τ_(RX i,l) may be the time delay value of the path RX iof the AN #l that is the calibration target, τ_(J) _(l) ^(RX) may be atime delay value from the point of S_(CAL) of the AN #l to the pointJ_(l), τ_(J) _(l) _(J) ₀ ^(RX) may be a time delay value from the pointJ_(l) to the point J₀, τ_(J) ₀ ^(RX) may be a time delay value from thepoint of S_(CAL) of the calibration device 350 to the point J₀, andτ_(CAL TX) may be a time delay value of the TX path of the calibrationdevice 350. The above-described τ_(J) _(l) ^(RX), τ_(J) ₀ ^(RX), andτ_(J) _(i) _(-J) ₀ ^(RX) may be well-known fixed values, and τ_(CAL TX)may be a component value of a common path for measurement of τ_(i,l)^(RX), ∀i, ∀l. Accordingly, the value of τ_(RX i,l) may be derived fromthe measured value of τ_(i,l) ^(RX).

In addition, the phase measurement value ϕ_(i,l) ^(RX) of the receptionpath calibration signal received through the path RX i (1≤i≤N_(l)) ofthe AN #l may be expressed as Equation 5 below according to thereception path of the transmission path calibration signal.ϕ_(i,l) ^(RX)=ϕ_(RX i,l)+ϕ_(J) _(l) ^(RX)+ϕ_(J) _(l) _(-J) ₀ ^(RX)+ϕ_(J)₀ ^(RX)+ϕ_(CAL TX)  [Equation 5]

In Equation 4, ϕ_(RX i,l) may be the phase characteristic value of thepath RX i of the AN #l that is the calibration target, ϕ_(J) _(l) ^(RX)may be a phase characteristic value from the point of S_(CAL) of the AN#l to the point J_(l), ϕ_(J) _(l) _(-J) ₀ ^(RX) may be a phasecharacteristic value from the point J_(l) to the point J₀, ϕ_(J) ₀ ^(RX)may be a phase characteristic value from the point of S_(CAL) of thecalibration device 350 to the point J₀, and ah ϕ_(CAL TX) may be a phasecharacteristic value of the TX path of the calibration device 350. Theabove-described ϕ_(J) _(l) ^(RX), ϕ_(J) ₀ ^(RX), and ϕ_(J) _(l) _(-J) ₀^(RX) may be well-known fixed values, and ah ϕ_(CAL TX) may be acomponent value of a common path for measurement of ϕ_(i,l) ^(RX), ∀i,∀l. Accordingly, the value of ϕ_(RX i,l) may be derived from themeasured value of ϕ_(i,l) ^(RX).

According to an exemplary embodiment of the present disclosure, τ_(i,l)^(RX) (1≤i≤N_(l)) and ϕ_(i,l) ^(RX) (1≤i≤N_(l)) for all the N_(l)reception paths of the AN #l may be parallelly measured through onereception path calibration signal transmitted by the calibrationcontroller 355. In such the exemplary embodiment, the calibration block324-2 of the AN #l may obtain average values of τ_(i,l) ^(RX) andϕ_(i,l) ^(RX) derived by the repeatedly-received reception pathcalibration signals, thereby minimizing the measurement errors.

In another exemplary embodiment of the present disclosure, measurementsmay be made for a part of the N_(l) reception paths through onereception path calibration signal transmitted by the calibrationcontroller. In this case, measurements for all the reception paths maybe completed through the repeated reception path calibration signals.The calibration block 324-2, which has completed the measurements forall the N_(l) reception paths of the AN #l, may determine the maximumtime delay measurement value τ_(l,max) ^(RX) and the maximum phasecharacteristic value ϕ_(l,max) ^(RX) as in Equation 6 below.

$\begin{matrix}{\tau_{l,\max}^{RX} = {\max\limits_{i}\tau_{i,l}^{RX}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

ϕ_(l, max )^(RX) = max_(i)ϕ_(i, l)^(RX)

The AN #l may report the two measured maximum values τ_(l,max) ^(RX) andϕ_(l,max) ^(RX) determined as described above to the calibrationcontroller 355 through a calibration report message (S570).

The calibration controller 355 of the calibration device 350 may performthe above-described procedure with each of the other ANs (S580). Aftercompleting mismatch measurement for a total of Σ_(i=1) ^(L) N_(l)reception paths for a total of the L ANs (or, a part of the L ANs)included in the serial fronthaul, the calibration controller 355 mayderive the maximum time delay value max_(i,m) τ_(RX i,m) and the maximumphase characteristic value max_(i,m) ϕ_(RX i,m). The calibrationcontroller 355 may calculate the time delay calibration value τ_(rx)^(l) and the time characteristic calibration value ϕ_(rx) ^(l) based onthe maximum time delay measurement value τ_(l,max) ^(RX) and the phasecharacteristic measurement value ϕ_(l,max) ^(RX) of the reception pathsof the AN #l as in Equation 7 below.

$\begin{matrix}{{\tau_{rx}^{l} = {{\max\limits_{i,m}\tau_{{RXi},m}} - {\max\limits_{i}\tau_{{RXi},l}}}}{\phi_{rx}^{l} = {{\max\limits_{m}\phi_{m,\max}^{RX}} - \phi_{l,\max}^{RX}}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

Thereafter, the calibration controller 355 may transmit, to thecalibration block 324-2 of the AN #l, a calibration adjustment messageincluding the time delay and phase characteristic calibration valuesτ_(rx) ^(l) and ϕ_(rx) ^(l) (S580). The time delay and phasecharacteristic calibration values τ_(rx) ^(l) and ϕ_(rx) ^(l) for thei-th reception path of the AN #l may be expressed as Equation 8 below.τ_(rx) ^(i,l)=τ_(rx) ^(l)+(τ_(l,max) ^(RX)−τ_(i,l) ^(RX))ϕ_(rx) ^(i,l)=ϕ_(rx) ^(l)+(ϕ_(l,max) ^(RX)−ϕ_(i,l) ^(RX))  [Equation 8]

Hereinafter, with respect to the procedures described through FIGS. 5Aand 5B, an operation procedure on the side of the calibration controller355 of the calibration device 350 and an operation procedure on the sideof the calibration block 324-2 of the AN #l will be described,respectively.

FIG. 6A is a flowchart illustrating an operation of a calibrationcontroller in the method of calibrating a mismatch of transmission pathsaccording to FIG. 5A.

Referring to FIG. 6A, the calibration controller 355 may determinewhether clock synchronization with ANs subjected to the transmissionpath mismatch calibration has been completed (S611), and may identify LANs and a set of N_(l) antennas/transceivers included in each AN (S612).

The calibration controller 355 may transmit a calibration commandmessage (AID=l, DIR=Tx) to the calibration block 324-2 of the AN #lamong the L ANs (S613). That is, the step S613 may correspond to thestep S510 in FIG. 5A.

Then, the calibration controller 355 may receive the transmission pathcalibration signals (i.e., TX CAL waveforms) sequentially transmittedfrom the calibration block 324-2 of the AN #l among the L ANs, and usethem to measure the time delay values and phase characteristic values ofthe transmission paths of the AN #l (S614). That is, the step S614 maycorrespond to the steps S521 to S524 in FIG. 5A.

Then, the calibration controller 355 may receive a calibrationcompletion report message from the calibration block 324-2 of the AN #lamong the L ANs (S615), and determine whether the time delay values andphase characteristic values of the transmission paths for all the L ANshave been measured (S616). As a result of the determination, if AN(s) tobe measured remain, the steps S613 to S615 may be repeated for theremaining AN(s), and if the measurement for all the AN(s) is completed,calibration values determined based on the measured time delay valuesand phase characteristic values to the ANs through a calibrationadjustment message (S617).

FIG. 6B is a flowchart illustrating an operation of a calibration blockin the method of calibrating a mismatch of transmission paths accordingto FIG. 5A.

Referring to FIG. 6B, the calibration block 324-2 may receive acalibration command message (AID=l, Dir=Tx) from the calibrationcontroller 355 (S621). That is, the step S621 may correspond to the stepS510 in FIG. 5A.

Then, the calibration block 324-2 may sequentially transmit TX CALwaveform signals to the calibration controller 355 at an interval ofΔ_(tx) through N_(l) TX blocks (S622, S623). That is, the steps S622 and623 may correspond to the steps S521 to S524 in FIG. 5A.

Then, the calibration block 324-2 may transmits a calibration completionreport message to the calibration controller 355 (S624), and may receivea calibration adjustment message including calibration values accordingto the time delay and phase characteristics of the transmission paths ofthe AN #l from the calibration controller 355 (S625).

Finally, the calibration block 324-2 may configure the mismatchadjustment blocks of the transmission paths corresponding to thereceived calibration values to complete the calibration of thetransmission paths (S627).

FIG. 7A is a flowchart illustrating an operation of a calibrationcontroller in the method of calibrating a mismatch of reception pathsaccording to FIG. 5B.

Referring to FIG. 7A, the calibration controller 355 may identify L ANssubjected to the reception path mismatch calibration and a set of N_(l)antennas/transceivers included in each AN (S711).

The calibration controller 355 may transmit a calibration commandmessage (AID=all, DIR=Rx, REP=R) to the calibration blocks 324-2 of theL ANs (S712). That is, in the calibration command message (AID=all,Dir=Rx, REP=R) may indicate that calibration targets are all the ANs,target paths are reception paths, and the number of repeatedtransmissions of the reception path calibration signal (i.e., RX CALwaveform) is set to R.

Then, the calibration controller 355 may transmit the reception pathcalibration signals to all the ANs (S713). The calibration controller355 may determine whether the reception path calibration signal has beentransmitted as many times as the number R of repeated transmissions setby the calibration command message (S714). If the reception pathcalibration signal has not been transmitted by the set number R ofrepeated transmissions, the calibration controller 355 may repeatedlyperform the steps S712 to S713 to additionally transmit the receptionpath calibration signals.

If the reception path calibration signals have been transmitted for theL ANs as many times as the set number R of repeated transmissions, thecalibration controller 355 may receive a calibration completion reportmessage including time delay values and phase characteristic values ofthe reception paths from the L ANs (S715).

Finally, the calibration controller 355 may transmit to the L ANs acalibration adjustment message including calibration values to which themeasured time delay values and the phase characteristic values arereflected (S716).

FIG. 7B is a flowchart illustrating an operation of a calibration blockin the method of calibrating a mismatch of reception paths according toFIG. 5B.

Referring to FIG. 7B, the calibration block 324-2 may receive acalibration command message (AID=all, DIR=Rx, REP=R) from thecalibration controller 355 (S721). That is, the step S721 may correspondto the step S550 in FIG. 5B.

Then, the calibration block 324-2 may receive the reception pathcalibration signal (i.e., RX CAL waveform) for N_(l) reception pathsfrom the calibration controller 355 (S722). That is, the step S722 maycorrespond to the step S561 in FIG. 5A. The calibration block 324-2 maydetermine whether the reception path calibration signals as many as theset number R of repeated transmissions have been received (S723), and ifthe reception path calibration signals have not been received by the setnumber R of repeated transmissions, the calibration block 324-2 mayrepeatedly perform the step 722 to additionally receive the receptionpath calibration signals.

Then, the calibration block 324-2 may determine the time delay value andthe phase characteristic value for each reception path, and may derivethe maximum value max_(i,m) τ_(RX i,m) among the time delay values andthe maximum value max_(i,m)ϕ_(RX i,m) among the phase characteristicvalues (S724).

The calibration block 324-2 may transmit to the calibration controller355 a calibration completion report message including the maximum timedelay value max_(i,m) τ_(RX i,m) and the maximum phase characteristicvalue max_(i,m) ϕ_(RX i m) (S725), and receive from the calibrationcontroller 355 calibration value according to the time delay values andphase characteristic values of the transmission paths of the AN #l fromthe calibration controller 355 through a calibration adjustment message(S726).

Finally, the calibration block 324-2 may configure the mismatchadjustment blocks of the reception paths corresponding to the receivedcalibration values to complete the calibration of the reception paths(S727).

Channel Reciprocity Calibration

When the calibration of the distributed transmission paths and receptionpaths is completed according to the procedures described with referenceto FIGS. 5A to 7B, the calibration controller 355 may be configured tocalibrate a transmission/reception mismatch of transceivers of aspecific AN. Such the mismatch calibration may be referred to as TDDchannel reciprocity calibration. Hereinafter, a TDD channel reciprocitycalibration operation of a specific AN through the calibrationcontroller will be described with reference to FIGS. 8A and 8B.

FIG. 8A is a sequence chart illustrating a method of performingcalibration for TDD channel reciprocity in a serial fronthaul accordingto an exemplary embodiment of the present disclosure.

Referring to FIG. 8A, interworking between the calibration controller355 and the calibration block 324-2 of the AN #l for the TDD channelreciprocity mismatch measurement and calibration is shown.

The calibration controller 355 may transmit a calibration commandmessage (AID=l, DIR=Tx/Rx) to the calibration block 324-2 of the AN #lthrough the data and control signal line 312 to indicate measurement ofa mismatch for TDD channel reciprocity (S811). Here, the DIR may be setto ‘Tx/Rx’ to indicate that the calibration command message is a TDDchannel reciprocity mismatch measurement command.

The calibration controller 355 may transmit a TDD channel reciprocitycalibration signal (i.e., TDD Ch. Rec. CAL waveform) through thecalibration transmission (CAL TX) path (S812). The TDD channelreciprocity calibration signal may be distributed to the N_(l) receptionpaths included in the AN #l through the calibration signal line 311 ofthe serial fronthaul 310, and detected by the calibration blocks 324-2.A reception signal gain C_(l) _(i) _(,CAL) ^(Rec) of the TDD channelreciprocity calibration signal received through the reception path RX i(1≤i≤N_(l)) of the AN #l may be expressed as in Equation 9 below.C _(l) _(i) _(,CAL) ^(Rec) =C _(RX i,l) *C _(J) _(l) *C _(J) _(l) _(-J)₀ *C _(J) ₀ *C _(CAL TX)  [Equation 9]

In Equation 9, C_(CAL TX) is a gain characteristic value of thecalibration transmission (CAL TX) path of the calibration device 350,C_(J) ₀ may be a gain characteristic value from the point of S_(CAL) ofthe calibration device 350 to the point J₀, C_(J) _(l) _(-J) ₀ may be again characteristic value from the point J_(l) to the point J₀, C_(J)_(l) may be a gain characteristic value from the point of S_(CAL) of theAN #l to the point J_(l), and C_(RX i,l) may be a gain characteristicvalue of the reception path i of the AN #l. The above-described C_(J)_(l) , C_(J) _(l) _(-J) ₀ , and C_(J) ₀ may be well-known gaincharacteristic values, and are also gain characteristic values of acommon path along with C_(CAL TX). The calibration block 324-2 of the AN#l may measure the gain characteristic values C_(l) _(i) _(,CAL) ^(Rec),i=1, . . . , N_(l) of the TDD channel reciprocity calibration signalreceived through the N_(l) reception paths, and may report thesemeasured values to the calibration controller 355 through the data andcontrol signal line 312 of the serial fronthaul by including them in acalibration report message (S813).

The calibration block 324-2 of the AN #l may sequentially transmit theTDD channel reciprocity calibration signals through N_(l) transmissionpaths (S814 to S817), and each TDD channel reciprocity calibrationsignal may be detected by the calibration controller 355 through thecalibration reception (i.e., CAL RX) path of the calibration device 350.A reception signal gain C_(CAL,l) _(i) ^(Rec) of the TDD channelreciprocity calibration signal, which is transmitted by the transmissionpath TX i (1≤i≤N_(l)) of the AN #l and received by the calibrationcontroller 355 through the calibration reception (i.e., CAL RX) path ofthe calibration device 350, may be expressed as in Equation 10 belowaccording to the transmission path.C _(CAL,l) _(i) ^(Rec) =C _(CAL RX) *C _(J) ₀ *C _(J) _(l) _(-J) ₀ *C_(J) _(l) *C _(TX i,l)  [Equation 10]

In Equation 10, C_(CAL RX) may be a gain characteristic value of thecalibration reception (i.e., CAL RX) path of the calibration device 350,C_(J) ₀ may be a gain characteristic value from the point of S_(CAL) ofthe calibration device 350 to the point J₀, C_(J) _(l) _(-J) ₀ may be again characteristic value from the point J_(l) to the point J₀, C_(J)_(l) may be a gain characteristic value from the point of S_(CAL) of theAN #l to the point J_(l), and C_(TX i,l) may be the gain characteristicvalue of the path TX i of the AN #l. The above-described C_(J) _(l) ,C_(J) _(l) _(-J) ₀ , and C_(J) ₀ may be well-known gain characteristicvalues, and may also be gain characteristic values of a common pathalong with C_(CAL RX) After transmitting the TDD channel reciprocitycalibration signal on the N_(l)-th transmission path, the calibrationblock 324-2 may report the completion of transmission of the TDD channelreciprocity calibration signals by transmitting a calibration completionreport message to the calibration controller 355 (S818). The calibrationcontroller 355 may use C_(l) _(l) _(,CAL) ^(Rec), i=1, . . . , N_(l),which are received through the calibration completion report message,and C_(CAL,l) _(l) ^(Rec), i=1, . . . , N_(l), which are identified bythe measurements, to calculate gain calibration values C_(Rec) ^(i,l),i=1, . . . , N_(l) for the N_(l) transceivers included in the AN #l asin Equation 11 below.

$\begin{matrix}{{C_{Rec}^{i,l} = {\frac{C_{{TXi},l}}{C_{{RXi},l}} = {\frac{C_{{CAL},l_{i}}^{Rec}}{C_{l_{i},{CAL}}^{Rec}}\frac{C_{CALRX}}{C_{CALTX}}}}},{i = 1},\ldots\mspace{14mu},N_{l}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

The gain calibration values for the N_(l) transceivers may betransmitted to the calibration block 324-2 through a calibrationadjustment message (S819), and the calibration block 324-2 may configurethe calibration values as values for adjusting the gains of thecorresponding mismatch adjustment blocks.

FIG. 8B is a flowchart illustrating an operation of a calibrationcontroller in a method of performing calibration for TDD channelreciprocity in a serial fronthaul according to an exemplary embodimentof the present disclosure.

Referring to FIG. 8B, a procedure for calibrating TDD channelreciprocity mismatches of all ANs connected in a serial fronthaul basedon the measurements on the TDD channel reciprocity mismatches of therespective ANs and the calculated calibration values therefor, whichwere described with reference to FIG. 8A is shown.

Referring to FIG. 8B, the calibration controller 355 may first determinewhether calibration of the transmission paths and reception paths of theAN #l connected in the serial fronthaul has been completed (S821).

If the calibration for the transmission paths and reception paths of theAN #l connected in the serial fronthaul has been completed, thecalibration controller 355 may transmit a calibration command message(ADI=l, DIR=Tx/Rx) to the calibration block 324-2 of the AN #l, andtransmit a TDD channel reciprocity calibration signal (i.e., TDD Ch.Rec. CAL waveform) to the calibration block 324-2 of the AN #l throughthe calibration transmission path (i.e., CAL TX) (S823).

The calibration controller 355 may receive a calibration report messagefrom the calibration block 324-2 of the AN #l to identify gainsC_(CAL,l) _(l) ^(Rec), i=1, . . . , N_(l) of the reception paths of theAN #l (S824).

The calibration controller 355 may receive the TDD channel reciprocitycalibration signal through the i-th transmission path of the AN #l, andmeasure the gain of the transmission path (S825). The step S825 may berepeated for the N_(l) transmission paths of the AN #l.

The calibration controller 355 may receive a calibration completionreport message from the calibration block 324-2 of the AN #l, calculatethe gain calibration values C_(Rec) ^(i,l), i=1, . . . , N_(l) of theN_(l) transmission paths of the AN #l (S727), and may transmit acalibration adjustment message including the calculated gain calibrationvalues to the AN #l (S728).

Hierarchical Calibration

FIG. 9 is a conceptual diagram illustrating a connection structure of aserial fronthaul according to an exemplary embodiment of the presentdisclosure.

Referring to FIG. 9, an exemplary embodiment in which each serialfronthaul connects one calibration device (i.e., CAL device) and aplurality of ANs, a plurality of serial fronthaul are connected to a CP,and a plurality of serial fronthaul are connected via a connector isshown. That is, one CP may be connected with a serial fronthaulincluding a first calibration device (i.e., CAL device 1), a serialfronthaul including a second calibration device (i.e., CAL device 2),and a serial fronthaul including a third calibration device (i.e., CALdevice 3). In particular, the serial fronthaul including the thirdcalibration device is connected to a serial fronthaul including a fourthcalibration device (i.e., CAL device 4) and a serial fronthaul includinga fifth calibration device (i.e., CAL device 5) through a connector. Inthis case, calibration should be performed for all distributed ANsconnected to the CP by the plurality of serial fronthaul links.Hereinafter, a procedure in which a plurality of calibration devicescooperate to calibrate distributed ANs will be described.

As one of calibration schemes for the system consisting of a pluralityof serial fronthaul links as shown in FIG. 9, one CP may serve as asingle calibration controller, and may use the methods proposed in FIGS.3 to 8B to directly calibrate mismatches between all RF paths includedin all ANs connected through the plurality of serial fronthaul links.Although the above method has the highest accuracy and reliability, as atime required for the mismatch calibration increases, scalability may beinsufficient.

Accordingly, in order to maximize scalability, a hierarchicalcalibration scheme may be considered. For example, after mismatchcalibration between the CP and the calibration devices (i.e., CALdevices 1 to 5) is first completed, the calibration devices may performmismatch calibration with ANs of the respective local fronthauls (i.e.,each calibration device performs mismatch calibration with ANs connectedto the fronthaul including the corresponding calibration device) inparallel. The above-described calibration method in consideration ofscalability will be described with reference to FIGS. 10 and 11.

FIG. 10 is a conceptual diagram illustrating a hierarchical calibrationmethod according to an exemplary embodiment of the present disclosure ina system including a plurality of serial fronthauls.

Referring to FIG. 10, a CP may identify a topology of serialfronthaul(s) included in a system through communication with all ANs andcalibration devices (i.e., CAL 1 to 5) connected through the serialfronthaul(s) (S1001). The CP may determine occurrence of an event (i.e.,mismatch calibration event) indicating that mismatch calibration isrequired such as expiration of a timer having a predetermined period ora change in the topology (S1002). When the mismatch calibration eventoccurs, the CP may use itself as a single calibration controller andperform a mismatch calibration procedure on the calibration devices CAL1 to 5 included in the plurality of serial fronthauls (S1003). After themismatch between all the calibration devices is calibrated through theabove procedure, each of the calibration devices CAL 1 to 5 may performmismatch calibration with the local ANs of the serial fronthaulincluding the corresponding calibration device (S1004). Theabove-described mismatch calibration between the calibration devices andtheir local ANs may be performed in parallel, so that the calibrationprocedure can be completed quickly.

FIG. 11 is a conceptual diagram illustrating a hierarchical calibrationmethod according to another exemplary embodiment of the presentdisclosure in a system including a plurality of serial fronthauls.

As shown in FIG. 9, the third calibration device (i.e., CAL device 3),the fourth calibration device (i.e., CAL device 4), and the fifthcalibration device (i.e., CAL device 5) may be connected through theconnector, and they may be connected to the CP through the serialfronthaul including the third calibration device (i.e., CAL device 3).

Referring to FIG. 11, the CP may identify the topology of the serialfronthaul(s) included in the system through communication with all ANsand calibration devices (i.e., CAL 1 to CAL 5) connected through theserial fronthauls (S1101). The CP may determine an occurrence of anevent (i.e., mismatch calibration event) indicating that mismatchcalibration is required such as expiration of a timer having apredetermined period or a change in the topology (S1102). When themismatch calibration event occurs, the CP may use itself as a singlecalibration controller and perform a mismatch calibration procedure onthe calibration devices CAL 1 to 3 included in the plurality of serialfronthauls directly connected with itself (S1003). Thereafter, the thirdcalibration device (i.e., CAL 3) directly connected to the CP may serveas a calibration controller to perform mismatch calibration with thefourth calibration device (i.e., CAL 4) and the fifth calibration device(i.e., CAL 5) connected through the connector (S1104).

On the other hand, the first to third calibration devices that havedirectly completed the mismatch calibration with the CP may performcalibration with the ANs of the respective local serial fronthauls(S1105), and the fourth and fifth calibration devices that havecompleted the mismatch calibration with the CP through the thirdcalibration device may also perform mismatch calibration with the ANs ofthe respective local serial fronthauls (S1106).

FIG. 12 is a block diagram illustrating a configuration of acommunication node according to exemplary embodiments of the presentdisclosure.

A communication node shown in FIG. 12 may be one of the calibrationcontroller, AN, or CP, which were described above.

Referring to FIG. 12, a communication node 1200 may comprise at leastone processor 1210, a memory 1220, and a transceiver 1230 connected tothe network for performing communications. Also, the communication node1200 may further comprise an input interface device 1240, an outputinterface device 1250, a storage device 1260, and the like. Therespective components included in the communication node 1200 maycommunicate with each other as connected through a bus 1270. However,each component included in the communication node 1200 may be connectedto the processor 1210 via an individual interface or a separate bus,rather than the common bus 1270. For example, the processor 1210 may beconnected to at least one of the memory 1220, the transceiver 1230, theinput interface device 1240, the output interface device 1250, and thestorage device 1260 via a dedicated interface.

The processor 1210 may execute a program stored in at least one of thememory 620 and the storage device 1260. The processor 1210 may refer toa central processing unit (CPU), a graphics processing unit (GPU), or adedicated processor on which methods in accordance with embodiments ofthe present disclosure are performed. Each of the memory 1220 and thestorage device 1260 may be constituted by at least one of a volatilestorage medium and a non-volatile storage medium. For example, thememory 1220 may comprise at least one of read-only memory (ROM) andrandom access memory (RAM).

The exemplary embodiments of the present disclosure may be implementedas program instructions executable by a variety of computers andrecorded on a computer readable medium. The computer readable medium mayinclude a program instruction, a data file, a data structure, or acombination thereof. The program instructions recorded on the computerreadable medium may be designed and configured specifically for thepresent disclosure or can be publicly known and available to those whoare skilled in the field of computer software.

Examples of the computer readable medium may include a hardware devicesuch as ROM, RAM, and flash memory, which are specifically configured tostore and execute the program instructions. Examples of the programinstructions include machine codes made by, for example, a compiler, aswell as high-level language codes executable by a computer, using aninterpreter. The above exemplary hardware device can be configured tooperate as at least one software module in order to perform theembodiments of the present disclosure, and vice versa.

While the embodiments of the present disclosure and their advantageshave been described in detail, it should be understood that variouschanges, substitutions and alterations may be made herein withoutdeparting from the scope of the present disclosure.

What is claimed is:
 1. A method for calibration in a serial fronthaul inwhich a calibration device and L (L is a natural number equal to orgreater than 1) access node(s) (AN(s)) are serially connected, themethod comprising: a step (a) of transmitting, by the calibrationdevice, to at least part of the L AN(s), a calibration command messageindicating calibration of transmission paths; a step (b) of determining,by the calibration device and the at least part of the L AN(s), timedelay values and phase characteristic values of the transmission pathsof the at least part of the L AN(s); and a step (c) of transmitting, bythe calibration device, to the at least part of the L AN(s), acalibration adjustment message indicating calibration of thetransmission paths based on the time delay values and the phasecharacteristic values of the transmission paths of the at least part ofthe L AN(s).
 2. The method according to claim 1, wherein the step (b)further comprises: a step (b-1) of transmitting, by an l-th AN of the atleast part of the L AN(s), a transmission path calibration signal to thecalibration device through each of N_(l) transmission paths included inthe l-th AN; a step (b-2) of determining, by the calibration device, atime delay value of an i-th transmission path among the N_(l)transmission paths based on a difference between a transmission timingof the transmission path calibration signal at the l-th AN and areception timing of the transmission path calibration signal at thecalibration device; and a step (b-3) of determining, by the calibrationdevice, a phase characteristic value of the i-th transmission path amongthe N_(l) transmission paths based on a difference between a phase ofthe transmission path calibration signal at the transmission timing anda phase of the transmission path calibration signal at the receptiontiming.
 3. The method according to claim 2, wherein the step (b) furthercomprises: a step (b-4) of reporting, by the l-th AN, time delay valuesand phase characteristic values of the N_(l) transmission paths to thecalibration device; and a step (b-5) of determining, by the calibrationdevice, the time delay values and the phase characteristic values of thetransmission paths of the at least part of the L AN(s) through the steps(b-1) to (b-4).
 4. The method according to claim 2, wherein thetransmission path calibration signals are sequentially transmitted at atime interval Δ_(tx) after a predetermined time Δ_(start) elapses from atransmission timing t₀ of the calibration command message.
 5. The methodaccording to claim 1, wherein the step (c) further comprises: a step(c-1) of determining, by the calibration device, a difference between amaximum time delay value among the time delay values of the transmissionpaths of the at least part of the L AN(s) and a time delay value of ani-th transmission path of an l-th AN of the at least part of the L AN(s)as a time delay calibration value for the i-th transmission path; a step(c-2) of determining, by the calibration device, a difference between amaximum phase characteristic value among the phase characteristic valuesof the transmission paths of the at least part of the L AN(s) and aphase characteristic value of the i-th transmission path of the l-th ANof the at least part of the L AN(s) as a phase characteristiccalibration value for the i-th transmission path; and a step (c-3) oftransmitting, by the calibration device, the time delay calibrationvalue and the phase characteristic calibration value of the i-thtransmission path to the l-th AN through a calibration adjustmentmessage.
 6. The method according to claim 1, wherein the serialfronthaul comprises: a timing clock signal line for clocksynchronization between the calibration device and the L AN(s); acalibration signal line for transferring a calibration signal betweenthe calibration device and the L AN(s); and a data and control signalline for transferring data and control signals for calibration betweenthe calibration device and the L AN(s).
 7. The method according to claim1, wherein the calibration device exists as a separate hardware device,one of the L AN(s) serves as the calibration device, or a centralprocessor (CP) connected to the L AN(s) through the serial fronthaulperforms a role of the calibration device.
 8. A method for calibrationin a serial fronthaul in which a calibration device and L (L is anatural number equal to or greater than 1) access node(s) (AN(s)) areserially connected, the method comprising: a step (a) of transmitting,by the calibration device, to at least part of the L AN(s), acalibration command message indicating calibration of reception paths; astep (b) of determining, by the calibration device and the at least partof the L AN(s), time delay values and phase characteristic values of thereception paths of the at least part of the L AN(s); and a step (c) oftransmitting, by the calibration device, to the at least part of the LAN(s), a calibration adjustment message indicating calibration of thereception paths based on the time delay values and the phasecharacteristic values of the reception paths of the at least part of theL AN(s).
 9. The method according to claim 8, wherein the step (b)further comprises: a step (b-1) of transmitting, by the calibrationdevice, a reception path calibration signal to an l-th AN of the atleast part of the L AN(s) through each of N_(l) reception paths includedin the l-th AN; a step (b-2) of determining, by the l-th AN, a timedelay value of an i-th reception path among the N_(l) reception pathsbased on a difference between a transmission timing of the receptionpath calibration signal at the calibration device, which is receivedthrough the i-th reception path, and a reception timing of the receptionpath calibration signal at the calibration device; a step (b-3) ofdetermining, by the l-th AN, a phase characteristic value of the i-threception path among the N_(l) reception paths based on a differencebetween a phase of the reception path calibration signal at thetransmission timing and a phase of the reception path calibration signalat the reception timing; and a step (b-4) of determining, by the l-thAN, the time delay values and the phase characteristic values of theN_(l) reception paths through the steps (b-1) to (b-3), and reportingthe time delay values and the phase characteristic values of the N_(l)reception paths to the calibration device.
 10. The method according toclaim 9, wherein when the calibration command message indicates a numberR of repeated transmissions of the reception path calibration signal,the reception path calibration signal is repeatedly transmitted R timesthrough the i-th reception path, and the time delay value and the phasecharacteristic value for the i-th reception path are determined based onR repeated transmissions of the reception path calibration signal. 11.The method according to claim 9, wherein the reception path calibrationsignals are sequentially transmitted at a time interval Δ_(rx) after apredetermined time Δ_(start) elapses from a transmission timing t₀ ofthe calibration command message.
 12. The method according to claim 8,wherein the step (c) further comprises: a step (c-1) of determining, bythe calibration device, a maximum time delay value among the time delayvalues of the reception paths of the at least part of the L AN(s) and amaximum phase characteristic value among the phase characteristic valuesof the reception paths of the at least part of the L AN(s); a step (c-2)of determining, by the calibration device, a difference between themaximum time delay value and a maximum time delay value of receptionpaths of an l-th AN of the at least part of the L AN(s) as a time delaycalibration value for the l-th AN; a step (c-3) of determining, by thecalibration device, a difference between the maximum phasecharacteristic value and a maximum phase characteristic value of thereception paths of the l-th AN of the at least part of the L AN(s) as aphase characteristic calibration value for the l-th AN; and a step (c-4)of transmitting, by the calibration device, the time delay calibrationvalue and the phase characteristic calibration value to the l-th ANthrough a calibration adjustment message.
 13. The method according toclaim 8, wherein the serial fronthaul comprises: a timing clock signalline for clock synchronization between the calibration device and the LAN(s); a calibration signal line for transferring a calibration signalbetween the calibration device and the L AN(s); and a data and controlsignal line for transferring data and control signals for calibrationbetween the calibration device and the L AN(s).
 14. The method accordingto claim 8, wherein the calibration device exists as a separate hardwaredevice, one of the L ANs serves as the calibration device, or a centralprocessor (CP) connected to the L AN(s) through the serial fronthaulperforms a role of the calibration device.
 15. A method for calibrationin a serial fronthaul in which a calibration device and L (L is anatural number equal to or greater than 1) access node(s) (AN(s)) areserially connected, the method comprising: a step (a) of transmitting,by the calibration device, to an l-th AN of the L AN(s), a calibrationcommand message indicating calibration of a time division duplexing(TDD) channel reciprocity; a step (b) of measuring, by the calibrationdevice and the l-th AN, gain characteristic values of N_(l) receptionpaths and N_(l) transmission paths included in the l-th AN; and a step(c) of determining, by the calibration device, a gain calibration valuefor each of the N_(l) reception paths based on the gain characteristicvalues of the N_(l) reception paths and the N_(l) transmission paths,and transmitting a calibration adjustment message including the gaincalibration value to the l-th AN.
 16. The method according to claim 15,wherein the step (b) comprises: transmitting, by the calibration device,to the l-th AN a TDD channel reciprocity calibration signal through theN_(l) reception paths included in the l-th AN; reporting, by the l-thAN, a reception signal gain measurement value of the TDD channelreciprocity calibration signal for each of the N_(l) reception paths;transmitting, by the l-th AN, to the calibration device a TDD channelreciprocity calibration signal through the N_(l) transmission pathsincluded in the l-th AN; and measuring, by the calibration device, areception signal gain measurement value of the TDD channel reciprocitycalibration signal for each of the N_(l) transmission paths.
 17. Themethod according to claim 16, wherein the TDD channel reciprocitycalibration signals are sequentially transmitted at a time intervalΔ_(tx) after a predetermined time Δ_(start) elapses from a transmissiontiming t₀ of the calibration command message or a timing of reportingthe reception signal gain measurement value.
 18. The method according toclaim 15, wherein the serial fronthaul comprises: a timing clock signalline for clock synchronization between the calibration device and the LAN(s); a calibration signal line for transferring a calibration signalbetween the calibration device and the L AN(s); and a data and controlsignal line for transferring data and control signals for calibrationbetween the calibration device and the L AN(s).
 19. The method accordingto claim 15, wherein the calibration device exists as a separatehardware device, one of the L ANs serves as the calibration device, or acentral processor (CP) connected to the L AN(s) through the serialfronthaul performs a role of the calibration device.
 20. The methodaccording to claim 15, wherein downlink channel information for the LAN(s) is estimated from uplink channel information for the L AN(s) usingthe TDD channel reciprocity.